The present invention relates to a developing device for supplying an electrostatic image with toner that is a visualizing material to develop the electrostatic image, the developing device being used in an image forming apparatus represented, for example, by an electrophotographic copying apparatus or a laser beam printer.
Known developing devices integrated into image forming apparatuses using an electrophotographic process are classified into a two-component system using carrier particles and toner and a one-component system using only toner.
The two-component developing device attaches the toner to carrier particles, carries it to an outer periphery of a developing sleeve to form a developer layer on the outer periphery of the developing sleeve, and brings the developer layer into contact with a surface of a photosensitive drum so that a Coulomb force of an electrostatic latent image previously formed on the photosensitive drum causes the toner to be separated from the carrier particles and then attached to the electrostatic latent image to develop the electrostatic image.
The one-component developing device forms a thin layer including only of the toner, on an outer periphery of the developing sleeve, opposes the photosensitive drum to the developing sleeve at a predetermined interval, and selectively supplies the toner to an electrostatic latent image formed on the photosensitive drum to develop this image.
Pseudo two-component developing devices have also been proposed which are actually one-component developing devices but which use a small amount of magnetic carrier.
Known pseudo two-component developing devices are disclosed in Jpn. Pat. Appln. KOKOKU Publication No. 7-40156 and Jpn. Pat. Appln. KOKOKU Publication No. 7-43554. These publications disclose an example having carrier particles accommodating area located below the developing sleeve to prevent the carrier particles, falling from the developing sleeve, from mixing into a toner holding section.
On the other hand, an example having an area located below the developing sleeve to hold the developer is disclosed in Jpn. Pat. Appln. KOKAI Publication. No. 9-6106. According to this publication, part of a developing housing is formed below the developing sleeve, the developing housing holds the developer supplied by a rotating arm rotated around a rotational center located below a rotational center of the developing sleeve.
The above described developing devices have a developer/toner conveying mechanism for conveying the developer or toner to the developing sleeve in order to form a developer layer or a toner layer on the developing sleeve.
Developer/toner conveying mechanisms are widely used which have a hopper above the developing sleeve so that the developer/toner is supplied to the developing sleeve based on free fall. After setup, even the two-component developing device is typically refilled only with the toner, which has been consumed to form images. The toner hopper will be described below.
Many toner hoppers include a toner holding section for accommodating the toner and an agitator for guiding the toner to the developing sleeve while agitating the toner in the toner holding section, to convey the toner to the developing sleeve based on the previously described free fall and conveyance of the toner carried out by the agitator.
The agitator extends parallel with an axis at the rotational center of the developing sleeve and has at least one fin-like toner agitating member made, for example, of a resin mold and extended in a radial direction orthogonal with the axis, the toner agitating member being slightly shorter than the radius (inner radius) of the toner holding section, and at least one sheet-like toner conveying member extended in a radial direction of the developing sleeve and which is longer than the inner diameter of the toner holding section, wherein the toner agitating member and the toner conveying member being arranged on the same rotating shaft so as to be rotated at a predetermined rotation speed.
However, many image forming apparatuses have been proposed in which the rotational center of the developing sleeve is located below the rotational center of the photosensitive drum. With these image forming apparatuses, it may be difficult to supply the toner to the developing sleeve based on the free fall, due to the arrangement of a system for conveying paper or another system.
Thus, examples have been proposed which include, in addition to the agitator, a toner conveying paddle or roller provided between the developing sleeve and the toner hopper to maintain (increase) the amount of toner supplied to the developing sleeve.
With the toner conveying paddle or roller, however, the developing device with the toner hopper in which the rotational center of the developing sleeve is located below the rotational center of the photosensitive drum may require the toner to be supplied even when a large amount of toner actually remains in the toner holding section in the toner hopper.
Alternatively, in the developing device using the agitator to convey the toner from the toner hopper to the developing sleeve, if a position where the toner is added is limited to a neighborhood of the center of an axis of the agitator (toner hopper) due to an ejection position of an auger conveyor, a large amount of time may be required to disperse the added toner in an axial (longitudinal) direction of the developing sleeve. This may make toner density uneven in the axial direction of the developing sleeve, resulting in an uneven density in a developed image.
Furthermore, when a large amount of supplied toner is present in one location within the toner hopper in the axial direction of the developing sleeve, a drive mechanism for rotating the agitator require a large torque, thereby increasing costs of the developing device.
It is an object of the present invention to provide an image forming apparatus that can prevent an uneven image density resulting from an insufficient toner supply to maintain a stable image density for a long time.
It is another object of the present invention to provide an image forming apparatus that can reduce the amount of toner, which may remain in a toner hopper.
It is yet another object of the present invention to provide an image forming apparatus that can stably supply toner to a developing roller.
The present invention provides a developing device for developing a latent image formed on an image bearing member by supplying the latent image with a developer that is a visualizing agent, the developing device comprising: a housing having a wall surface extended in a first direction parallel with the direction of a rotational axis of the image bearing member and having a cylinder or part thereof having its radius defined in a direction orthogonal with the first direction, the housing holding the developer supplied to the latent image formed on the image bearing member; a developing roller located below the rotational axis of the image bearing member in a vertical direction, having a rotational center parallel with the first direction, and formed so as to rotate around the rotational center; and an agitating member having a rotational center extended in the first direction and rotated within the housing along the wall surface, the agitating member having a sheet-like member with a plurality of fin-like portions arranged therein and which are polygons each having a first free length extending from the rotational center in a circumferential direction toward the wall surface of the housing and a second free length that is longer than the first free length, the fin-like portion having an opening formed in a portion thereof, the agitating member conveying the developer to the developing roller and dispersing the developer in the first direction for agitation when the sheet-like member is rotated along an inner wall of the housing.
The present invention also provides an image forming method comprising steps of: forming a latent image on an image bearing member; and selectively supplying toner to the latent image formed on the image bearing member to develop the latent image, using a developing device having a housing having a wall surface extended in a first direction parallel with the direction of a rotational axis of the image bearing member and having a cylinder or part thereof having its radius defined in a direction orthogonal with the first direction, the housing holding the developer supplied to the latent image formed on the image bearing member, a developing roller located below the rotational axis of the image bearing member in a vertical direction, having a rotational center parallel with the first direction, and formed so as to rotate around the rotational center, and an agitating member having a rotational center extended in the first direction and rotated within the housing along the wall surface, the agitating member having a sheet-like member with a plurality of fin-like portions arranged therein and which are polygons each having a first free length extending from the rotational center in a circumferential direction toward the wall surface of the housing and a second free length that is longer than the first free length, the fin-like portion having an opening formed in a portion thereof, the agitating member conveying the developer to the developing roller and dispersing the developer in the first direction for agitation when the sheet-like member is rotated along an inner wall of the housing.
The present invention further provides an image forming apparatus comprising: a developing device including: a photosensitive body on which a latent image corresponding to image information to be output is formed; a developing magnet having N and S poles alternately arranged inside, the developing magnet being rotated; a sleeve formed so as to rotate around an outer periphery of the developing magnet to supply an image bearing member with toner to be supplied to the latent image formed on the photosensitive body; a toner hopper constituting part of the housing, provided below a rotational center of the sleeve at a predetermined interval in a vertical direction, and having a table section for temporarily holding toner supplied to the sleeve in order to accommodate the toner supplied to the sleeve; and an agitating member having a rotational center extended in the first direction and rotated within the housing along the wall surface, the agitating member having a sheet-like member with a plurality of fin-like portions arranged therein and which are polygons each having a first free length extending from the rotational center in a circumferential direction toward the wall surface of the housing and a second free length that is longer than the first free length, the fin-like portion having an opening formed in a portion thereof, the agitating member conveying the developer to the developing roller and dispersing the developer in the first direction for agitation when the sheet-like member is rotated along an inner wall of the housing, the developing device developing the latent image formed on the photosensitive body by selectively supplying the latent image with the toner, the image forming apparatus further including a transfer device for transferring the toner formed on the photosensitive body by the developing device, to a transfer object.